IsNobody throws NRE

[BaterPL]

Version Information
Version of Akka.NET? v1.4.24
Which Akka.NET Modules? Akka

Describe the bug
After changes in IsNobody implementation, our tests started failing because of NRE. It seems that the new implementation assumes that Path is not null which might not be true. In our case, it's caused by the mock of IActorRef.

To Reproduce
Steps to reproduce the behavior:

1. Prepare a test where you mock IActorRef using some mocking library (e.g. NSubstitute)
2. Call IsNobody() on mock
3. NRE is thrown

Code sample: https://github.com/BaterPL/akka-isNobody-example/blob/main/src/IsNobodyTests.cs

Expected behavior
IsNobody returns true instead of throwing an exception

Actual behavior
NullReferenceException

Environment
Windows, .NET 5

[Aaronontheweb]

Thanks - we should patch this

[jlaur]

See #5114

[Aaronontheweb]

closed via #5220